i) Overview
The Fischer-Tropsch (FT) process for converting carbon monoxide and hydrogen to liquid motor fuels and/or wax has been known since the 1920's.
During the Second World War synthetic diesel was manufactured in Germany using coal gasification to supply a 1:1 ratio of hydrogen and carbon monoxide for conversion to fuel hydrocarbons. Because of trade sanctions and the paucity of natural gas, South Africa further developed the coal via gasification route to synthesis gas and employed a fixed-bed iron Fischer-Tropsch catalyst. Iron catalysts are very active for the water-gas shift reaction which moves the gas composition from a deficiency of hydrogen and closer to the optimum H2/CO ratio of around 2.0. When large natural gas supplies were developed, steam and autothermal reformers were employed to produce the synthesis gas feedstock to slurry-bed FT reactors using cobalt or iron catalysts.
In Gas-To-Liquids (GTL) plants, compromises must be made between liquid product yield and plant operating and capital costs. For example, if there is a market for electricity, a steam reformer design may be chosen because this technology produces a large amount of waste heat: flue gas heat can be converted to electricity using an ‘economiser’ and steam turbine. If conservation of natural gas feedstock and low capital cost are paramount, autothermal or partial oxidation reformers using air are favored.
Another factor in selecting the best reformer type is the nature of the reformer hydrocarbon feed gas. If the gas is rich in CO2, this can be advantageous because the desired H2/CO ratio can then be achieved directly in the reformer gas without the need to remove excess hydrogen, and some of the CO2 is converted to CO, increasing the potential volume of liquid hydrocarbon product that can be produced. Additionally, the volume of steam that is required is reduced, which reduces the process energy requirements.
The market for Fischer-Tropsch (FT) processes is concentrated on large “World-Scale” plants with natural gas feed rates of greater than 200 million scfd because of the considerable economies if scale. These plants operate at high-pressure, about 450 psia, and use extensive recycling of tail gas in the FT reactor. For, example, the Norsk Hydro plant design has a recycle ratio of about 3.0. The emphasis is on achieving the maximum wax yield. In terms of product slate, these large plants strive for the maximum yield of FT waxes in order to minimize the formation of C1-C5 products. The waxes are then hydrocracked to primarily diesel and naphtha fractions. Unfortunately, light hydrocarbons are also formed in this process. The reformers use some form of autothermal reforming with oxygen produced cryogenically from air, an expensive process in terms of operating cost and capital cost. The economies of scale justify the use of high operating pressure, the use of oxygen natural gas reforming, extensive tail gas recycling to the FT reactor for increasing synthesis gas conversion and controlling heat removal and product wax hydrocracking. To date, an economical FT plant design has not been developed for small plants with capacities of less than 100 million scfd.
The present invention strives for optimized economics in a completely different market: small FT plants using less than 100 million scfd. The emphasis is on simplicity and minimized capital cost, somewhat at the expense of efficiency.
ii) Existing FT TechnologiesTechnology of this inventionLarge plants, >25 MMscfdSmall plants, <100 MMscfdHigh pressure, >200 psiaLow pressure, <200 psiaOxygen to reformerAir to reformerExtensive recycling to FT reactorNo recycling (“once-through”or reformerprocess)Low single-pass FT COHigh single-pass conversionconversion (<50%)(>65%)Deliberate and extensive wax formationLess than 10% wax formationHydrocracking waxesNo hydrocracking operationsMultiple-pass FT reactorsSingle-pass-FT reactorLow FT diesel yield (<50%)High diesel yield (55-90% ofhydrocarbon liquid)iii) Prior Art
The catalytic hydrogenation of carbon monoxide to produce a variety of products ranging from methane to heavy hydrocarbons (up to C80 and higher) as well as oxygenated hydrocarbons is usually referred to as Fischer-Tropsch synthesis. The high molecular weight hydrocarbon product primarily comprises normal paraffins which can not be used directly as motor fuels because their cold properties are not compatible. After further hydroprocessing, Fischer-Tropsch hydrocarbon products can be transformed into products with a higher added value such as diesel, jet fuel or kerosene. Consequently, it is desirable to maximize the production of high value liquid hydrocarbons directly to that component separation or hydrocracking are not necessary.
Catalytically active group VIII, in particular, iron, cobalt and nickel are used as Fischer-Tropsch catalysts; cobalt/ruthenium is one of the most common catalyzing systems. Further, the catalyst usually contains a support or carrier metal as well as a promoter, e.g., rhenium.
Metal oxides, e.g., silica, alumina, titania, zirconia or mixture thereof, have been utilized as catalyst supports in Fischer-Tropsch hydrocarbons synthesis. U.S. Pat. No. 4,542,122 disclosed a cobalt or cobalt thoria on titania as a hydrocarbon synthesis catalyst. U.S. Pat. No. 4,088,671 disclosed a cobalt-ruthenium catalyst where alumina was used as a support. European Pat. No. 142,887 described a silica supported cobalt catalyst together with zirconium, titanium, ruthenium and/or chromium.
U.S. Pat. No. 4,801,573 described a promoted cobalt and rhenium catalyst supported on alumina. The amount of cobalt is most preferably about 10-40 wt % of the catalyst. However, rhenium is preferably about 2-20 wt of cobalt content. U.S. Pat. No. 5,248,701 disclosed a copper promoted cobalt-manganese spinel that was said to be useful as a Fischer-Tropsch catalyst with selectivity for olefins and higher paraffins.
U.S. Pat. No. 4,738,948, issued in Apr. 19, 1988, describes a catalyst comprising cobalt ruthenium at an atomic ratio of 10-400, on a refractory carrier, such as titania or silica. The catalyst is used for conversion of synthesis gas with an H2:CO ratio of 0.5-10, preferably 0.5-4, to C5-C40 hydrocarbons at a pressure of 80-600 psig and at a temperature of 160-300° C., at a gas hourly space velocity of 100-5000 v/hr/v.
U.S. Pat. No. 2003/0134912 A1 published Jul. 17, 2003 describes a Fischer-Tropsch process that uses an activated carbon support to limit formation of heavier components by size exclusion. The product is segregated into three (3) boiling fractions, which are then re-combined to make diesel. The diesel is not made as a direct product exiting the Fischer-Tropsch reactor.
U.S. Pat. No. 4,801,573, issued Jan. 31, 1989 (expired), described a catalyst of cobalt and rhenium deposited on alumina. The reaction products are a complicated mixture that follow the Shulz-Flory distribution.
After a period of time in operation, a catalyst becomes deactivated, losing its effectiveness for synthesis gas conversion. Among the main deactivation mechanisms for cobalt based catalysts are sulfur poisoning [e.g. R. L. Espinoza, et al, Applied Catalysis A:General 186 (1999)13], metal oxidation [e.g. D. Schanke et al, Catal. Lett. 34 (1995) 269] and surface condensation of heavy hydrocarbons [e.g. E. Iglesia et al, J. Catal. 143 (1993)345].
U.S. Pat. No. 5,728,918, issued on Mar. 17, 1998, described a catalyst comprising cobalt on a support, used for conversion of synthesis gas with an H2:CO ratio of 1-3, preferably 1.8-2.2, to C5+ hydrocarbons at a pressure of 1-100 bar and at a temperature of 150-300° C., at a typical gas hourly space velocity of 1000-6000 v/hr/v. Generation Activation of this catalyst was achieved by using a gas containing carbon monoxide and less than 30% hydrogen at a temperature more than 10° C. above Fischer-Tropsch conditions in the range 100-500° C. at a pressure of 0.5-10 bar, for air, at least 10 min preferably 1-12 hours.
U.S. Pat. No. 2,471,288 refers to the use of a alumina as a catalyst for cracking gas oil, while a Polish paper published in Przemysl Chemiczny (2004), 83 (3) pp. 137-140 describes the use of CoAl2O3 for the purpose of cracking propane. Clearly, cobalt and alumina have paraffin cracking activity.
Fischer-Tropsch synthesis performed at low pressure, 17-21 atmospheres, and relatively high temperature, usually produces short chain hydrocarbons of 0.6-0.7 chain growth probability factor. U.S. Pat App. No. 20050209348 published on Sep. 22, 2005, described a Fischer-Tropsch process performed at an elevated temperature between 230-280° C., for example 240° C. and at elevated pressure typically between 1.7 MPa and 2.1 MPa, for example 1.8 MPa, using a compact reactor. The preferred catalyst comprised a coating of gamma alumina support with 10-40% cobalt (by weight compared to the alumina) and with a promoter such as ruthenium, platinum or gadolinium which is less than 10% of the cobalt weight. The gas hourly space velocity was very high, for example 20000 hr−1 and the produced hydrocarbon liquid consisted of saturated linear alkanes of chain lengths range between about 6-17. Consequently, it is rich in aircraft fuel. However, the selectivity to the production of C5+ hydrocarbon was less than 65% and the conversion of carbon monoxide was no greater than 75%.
Hence, there is still a great need to identify other Fischer-Tropsch processes which can be used to directly produce different types of fuel such as, diesel fuel.